Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Defined Benefit Plan Disclosure [Line Items]
Service cost, net	$ 4,016	$ 4,093	$ 8,208	$ 8,258
Interest cost on projected benefit obligations	3,208	3,165	6,508	6,346
Expected return on plan assets	(9,420)	(9,313)	(19,139)	(18,382)
Recognition of prior service cost	(1,820)	(1,169)	(3,707)	(3,187)
Recognition of actuarial losses/(gains)	6,507	6,223	13,205	13,280
Net periodic pension cost/(credit)	2,491	2,999	5,075	6,315
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost, net	272	141	544	282
Interest cost on projected benefit obligations	1,061	1,094	2,122	2,188
Expected return on plan assets	(1,732)	(1,684)	(3,464)	(3,368)
Recognition of prior service cost	0	0	0	0
Recognition of actuarial losses/(gains)	1,451	1,639	2,902	3,278
Net periodic pension cost/(credit)	1,052	1,190	2,104	2,380
Other Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost, net	3,744	3,952	7,664	7,976
Interest cost on projected benefit obligations	2,131	2,053	4,354	4,122
Expected return on plan assets	(7,688)	(7,629)	(15,675)	(15,014)
Recognition of prior service cost	(1,727)	(974)	(3,521)	(2,797)
Recognition of actuarial losses/(gains)	5,369	5,058	10,929	10,950
Net periodic pension cost/(credit)	1,829	2,460	$ 3,751	5,237
Defined Benefit Plan, Expected Future Employer Contributions, Current Fiscal Year, Description			25900
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost, net	0	0	$ 0	0
Interest cost on projected benefit obligations	16	18	32	36
Expected return on plan assets	0	0	0	0
Recognition of prior service cost	(93)	(195)	(186)	(390)
Recognition of actuarial losses/(gains)	(313)	(474)	(626)	(948)
Net periodic pension cost/(credit)	$ (390)	$ (651)	$ (780)	$ (1,302)